fn main() {
    let is_fun: bool = false;
    let num = -10; // i32 -> signed integer of 32bits

    // u8 -> unsigned integer of 8bits
    // 2^8-1 -> 255 -> the max of 8bit
    let small_num: u8 = 10;

    // i8 -> -2^7 -> 2^7-1
    // -128 -> 127 // 1位用于表示符号，所以是127
    let small_num_2: i8 = 127;

    // operating system related type
    let sys_num: isize = -10;
    let sys_num_2: usize = 10;
}
